This invention relates generally to leak testers for moulded plastic containers.
In the blow moulding art, for example, it is normal to route containers leaving a blow moulding machine through a leak tester in which each container is pressurized with air to test for leaks.
Since the containers have some elasticity, the pressure in the container will rise to a peak as the container expands slightly under pressure, and then drop off as the container relaxes. If there is no leak, the pressure in the container will stabilize at a value lower than the peak value. The period of time required for the pressure to stabilize is determined for the particular containers being tested and the pressure in each container is monitored for that period, from initial pressurization. If the pressure is below the defined level at the end of the period, the container has a leak and is rejected.
In a typical conventional leak tester, the containers are conveyed in a part-circular path or loop for testing and are connected to a central source of pressurized air by way of a series of air hoses. Each hose is coupled to the source via a rotary coupling and extends to a test head that is clamped onto the mouth of the container being tested. In a typical conventional leak tester, there may be thirty test heads each with its own associated hose.
Once a container is pressurized, an associated fill valve is closed and the pressure in the container is measured during the test cycle. Each test head may have a pressure transducer and an individual processor to determine whether the container leaks. The leak data can be transmitted to a central processor via non-contact means, such as optical signals or via rotary commutator rings. The individual test heads remain connected to central sources of both power and air throughout the test process.
It will be appreciated from the foregoing description that conventional leak testers are quite complex and expensive. Not only that, but the leak tester may impose a limit on the through-put of containers being produced, since each container must remain in the leak test loop for a period of time sufficient to complete the pressure test. As an example, a prior art leak tester may be capable of handling a maximum of approximately 300 containers per minute.
An object of the present invention is to provide a leak tester which addresses at least some of these shortcomings of the prior art.
In one aspect, the invention provides a method of leak testing moulded plastic containers in which the containers are conveyed seriatim along a path. Each successive container is pressurized at a first station along the path using a defined source pressure and is then sealed to maintain pressurization. The container is then disconnected from the source. A test station is provided a distance downstream along the path from the first station selected to correspond to the time required to allow the pressure in the container to stabilize if there is no leak. Each sealed container is conveyed from the first station to the test station and the pressure in the container is momentarily determined at the test station. A signal is provided if the pressure in the container is below a defined value, indicating a leak.
A corresponding leak testing apparatus includes a conveyor for transporting the containers seriatim along a path. A pressurization unit is provided at a first station along said path and includes a defined pressure source. The apparatus also includes a plurality of self-contained test heads, each of which is adapted to sealingly engage the mouth of a said container, and means for bringing the test heads successively into said sealing engagement with the mouths of respective containers approaching said first station, and for successively disengaging the test heads from the containers at a test station spaced downstream from said first station a distance selected to allow pressure in each container to stabilize if there is no leak in the container. Each test head is adapted to momentarily engage the pressure source at the first station for pressurizing an associated container, and includes means for sealing the container against pressure loss following disengagement from said pressure source. Each head further includes a pressure transducer for determining the pressure in the container, means for signalling pressure lower than a defined value, and battery supply power means. The test head is normally de-energized but can be remotely energized momentarily when pressure in the container is to be measured. At the test station, the apparatus includes means for remotely energizing each test head so the said signalling means will provide an indication if the measured pressure is lower than said defined value.
The method and apparatus of the invention provide a number of significant advantages over prior art leak testers. A primary advantage is that the leak tester does not restrict the speed of container production. The test station can be positioned downstream from the first (air pressurization) station at whatever distance is required to allow time for the pressure within the container to stabilize and be measured. In prior art systems, the speed of travel of the containers must be restricted so that the containers remain within the loop of the leak tester for sufficient time to complete testing.
In the invention, there is no need for air hoses connecting the containers to the pressure source, or power supply wiring. The containers are momentarily connected to the pressure source at the first station and then sealed. The test heads are battery powered so that no wiring is required. The heads are energized only momentarily when a pressure reading is to be taken, so that battery life can be maximized.
Preferably, the test path is at least generally linear so that several leak tester xe2x80x9clinesxe2x80x9d can be installed side-by-side in minimum space.